1. Field of the Invention
The present invention relates to a method for laminating glass sheets and other frangible materials, wherein a plastic film is sandwiched between sheets.
Flat or non-flat sheets of glass, ceramics, polymers, or combinations of these materials may be laminated in accordance with the teachings of the present invention.
2. Discussion of the Prior Art
Laminates provide a way of strengthening frangible material, for example glass, so as to extend its uses and to render it safer to use in certain circumstances. Thus laminated glass products can be used for automotive and aircraft glazing, glass doors, balustrades, bulletproofing and many other uses where the glass product must be strong and/or shatterproof.
In conventional laminated glass products a sheet of glass is bonded to a layer of polymer adhesive film, and a further sheet or layer of material is bonded to the other side of the adhesive film layer, so that the adhesive film is sandwiched between two outer layers. If the glass sheet is then struck a blow it cracks or breaks, but does not shatter into small hazardous sharp pieces as the broken pieces are still bonded to and held in place by the polymer layer. If the laminated glass is used in a car windscreen, therefore, occupants of the car are not showered with broken glass upon breakage of the windscreen.
A number of methods for producing such laminates have been disclosed. For Example, see U.S. Pat. Nos. 5,268,049; 5,118,371; 4,724,023; 4,234,533; and 4,125,669. Laminated glass has been generally manufactured by a process wherein a stack of at least two sheets of glass having a plastic film called an intermediate film or laminating film, typically a plasticized polyvinyl butylal (PVB) film, is sandwiched between each pair of adjacent sheets of glass which is subjected to evacuation, pressing and heating.
Usually this involves long heating under temperatures of around 80° C.-140° C. and high pressure, 4 MPa-20 MPa. The main problem encounter is that air is trapped between the film and glass surfaces, which air must be removed. This is required to prevent the laminate from bubbling. Removing the remainder of the air requires long heating and high pressure. The bubbling is a visible and objectionable defect that in most cases is absolutely unacceptable. Besides, bubbling within the laminate may reduce its strength in this area and cause delamination.
At the same time removing air is not an easy task because it is trapped between both sides of the plastic film and glass sheet and there are only two mechanisms by which the air can escape: diffusion and dissolving in the film. Both processes are very slow, requiring long term heating and applying of high pressure. The bigger the glass sheet, the longer the time required. An especially long time is required for making multi-layer laminates. As a result, the productivity of such processes is low and they require considerable capital expenditure to set up the necessary costly apparatus such as autoclaves.
Many prior art patents focus on the solution of problems related to the air escaping. In U.S. Pat. No. 5,268,049, glass sheets are spaced apart, and in the method described by U.S. Pat. No. 5,268,049, a liquid resin is used. In U.S. Pat. No. 4,234,533 the two sheets are held at an angle and in U.S. Pat. No. 5,118,371 the thickness of PVB gradually increases (or decreases) from the one side to the other side of the glass sheets. In U.S. Pat. No. 3,509,015 a method is described for producing laminated glass by sealing the periphery of two parallel glass sheets with pressure sensitive tape and forcing resinous material under pressure into the inter-sheet space. The resinous material is forced through a self-closing valve held in place with the tape while trapped air escapes through an aperture in the taped seam at the top of the cell. U.S. Pat. No. 4,125,669 describes a similar method in which two glass panes are sealed all around except for a filling opening and an aeration opening, and a binder material is introduced into the envelope thus formed in an amount calculated to exactly fill the envelope. Putty is applied to the openings just before emergence of the binder upon laying the filled envelope flat.
U.S. Pat. No. 3,315,035 describes a method involving the maintaining of the glass sheets in opposite relationship, heating the sheets to about 200° F. and injecting a resin composition containing a hardening agent, preheated to about 200° F., into the inter-sheet space and curing the assembled article. In U.S. Pat. No. 4,234,533 the seal around the sheets is formed by a gas-permeable, resin-impermeable material such as “Scotchmount”. In some inventions (see for example U.S. Pat. Nos. 4,828,598 and 4,724,023) the laminating process is conducted in a vacuum. The vacuum environment helps air to escape and, in general, can reduce the level of trapped air. However, heating in a vacuum is always difficult, inefficient and therefore the laminating process still requires a long time.
Thus, all the above described methods of air bubble removal, are not fully effective and still require long term heating (high energy consumption) and special expensive equipment, such as high pressure autoclaves.
At the same time, extremely large numbers of windshields, windows and other laminate products are made each year. Accordingly, there is a clear need in the art for a more effective and less expensive method for laminating glass sheets which eliminates expensive equipment and reduces energy consumption.